Enormous amounts of waste heat are generated by a wide variety of industrial and commercial processes and operations. Example sources of waste heat include heat from space heating assemblies, steam boilers, engines, and cooling systems. With the use of low grade waste heat, such as waste heat having a temperature of below about 400 degrees Fahrenheit (about 204 degrees Celsius) for example, conventional heat recovery systems generally do not operate with sufficient efficiency to make the recovery of energy cost effective. The net result is that vast quantities of waste heat are simply dumped into the atmosphere, the ground, the water, or other types of surroundings without producing useful work.
In one conventional method to generate electricity from waste heat, a two-cycle system may be used in heat recovery applications with waste heat sources of different temperature levels. In such two-cycle configurations, the hot heat source heats a high boiling point liquid in a top loop and the cold heat source heats a low boiling point liquid in a separate bottom loop. Because these two-cycle systems are complex and require multiple components, however, the overall costs of typical two-cycle systems may be relatively high.
In another conventional system provided to generate electricity from waste heat, a cascaded organic rankine cycle system may be used. The cascaded organic rankine cycle includes a pair of organic rankine cycle systems. The cycles are combined and the respective organic working fluids are chosen such that the organic working fluid of the first organic rankine cycle is condensed at a condensation temperature that is above the boiling point of the organic working fluid of the second organic cycle. A single common heat exchanger may be used as both the condenser of the first organic rankine cycle system and the evaporator of the second organic rankine cycle. A cascaded organic rankine cycle system thus efficiently converts surplus waste heat into electricity within certain temperature ranges.
There is thus a desire therefore for an improved organic rankine cycle system or other type of waste heat system that effectively recovers waste heat over a wide temperature range from multiple low grade heat sources at different operating conditions. For example, the waste heat of one or more charge air coolers may be incorporated into an organic rankine cycle system that otherwise would be dissipated without producing useful work. Such a system would efficiently use the waste heat for increasing net power output.